Supervisory system for telegraph circuits



Dec. 26, 19.44. R BHEA 2,365,816

SUPERVISORY SYSTEM FOR TELEGRAPH CIRCUITS Filed Feb. 18, 1945 5 Sheets-Sheet l A'AvA' POLAR/ZED lNl/EN TOR R B. HE/1 RN ATTORNEVI Fla 2 Dec. 26, 1944.

POLAR/Z5 R. B. HEARN 2,365,816

SUPERYISORY SYSTEM FOR TELEGRAPH CIRCUITS Filed Feb. 18, 1943 5 Sheets-Sheet 2 POLAR/ZED illHl' HIGH RES/57' POLAR/2E POLAR! ED lNl/E/VTOR R. B. HEARN Dec. 26, 1944. R HEARN 2,365,816

SUPERVISORY SYSTEM FOR TELEGRAPH CIRCUITS Filed Feb. 18, 1943 5 Sheets-Sheet 3 POLAR/2E OPERATORS COMMON POSEOU.

FIG. 3

lNl/ENTOR y R B. HEARN A TTORNEV Dec. 26, 1944. RB. HEARN 2,365,816

SUPERVISORY SYSTEM FOR TELEGRAPH CIRCUITS Filed Feb. 18, 1943 5 Sheets-Sheet 4 POLA RIZED as Q 2 g I a I '11 I MMA! PIE}, &

)NVENTOR By R. a HEARN ATTO'P/VEV Dec. 26, 1944. R: B. HEARN SUPERVISORY SYSTEM FOR TELEGRAPH CIRCUITS Filed Feb. 18, 1943 5 Sheets-Sheet 5 QMNEYEQ v, E N R T T A Patented Dec. 26, i944 UNITED STATES PATENT OFFICE SUIERVISORY SYSTEM FOR TELEGRAPH CIRCUITS Richard B. Hearn, Hollis, N. Y., assignor to Bell Telephone Laboratories,

Incorporated, New

This invention relates to telegraph systems and particularly to repeaters through which two stations may be interconnected for communication between the stations.

It has sometimes been the practice in telegraph systems involving switching operations at a cen.- tral station to provide for signal transmission between a subscriber station and the line terminating repeater of the subscriber line at the switching station on a basis which is polar in one direction and differential in the other direction. A transmission system of this type is shown in Patent 2,143,000 granted January 10, 1939, to W. W. Cramer et al. In the arrangement shown in the foregoing patent, the transmission is polar from the subscriber station to the line terminating repeater, the transmission of signals being accomplished by operation of a pole changer relay system which connects either positive or negative po- '1arity between the telegraph line and ground, the

potential being the same for both polarities. At the line terminating repeater, th line is connected to ground during idle intervals and during transmission from the subscriber station, and the relay in the repeater which receives signals from a the subscriber station and retransmits them is polar. The relay in the line terminating repeater which repeats signals toward the subscriber station operates to supply the ground connection for one signaling condition and for the other COUCH-("3 tion supplies a signaling potential of substantially twice the value of that at the subscriber station and in series-aiding relation to the source of signaling potentialat the subscriber station as connected by the pole changer relay system when, the subscriber station is receiving signals, so that signaling current for transmission to the subscriber station is unidirectional and for one signaling condition is substantially three times as great as the signaling current for the other signaling condition. The repeater relay which reto the greater of the two signaling currents which it receives from the line terminatin repeater when signals are being transmitted to the subscriber station. -Due to this biasing of the receiving relay so that it distinguishes between the two values of signaling current it does not follow signals transmitted from the subscriber station,

since one signaling condition generated at the subscriber station ineffectively opposes the bias and the other supplements the bias.

It has been found that when line terminating repeaters arranged to afford signal transmission as described above are interconnected, as through a cord circuit, to provide for intercor'nmunication between two subscriber stations, and the attendants at the two subscriber stations perform disconnect operations simultaneously' or in rapid succession, a condition of mutual oscillatory interaction may arise between the two line tern 1i nating repeaters which prevents the disconnect signal initiated by either of the two subscriber stations from becoming efiective at the switching station. This condition may also arise when the circuit extending between the subscriber stations includes more than one switching station.

Accordingly, it is an object of the present invention to estop oscillatory interaction between line termination repeaters included in a communication circuit extending between two stations when the two stations initiate like signaling conditions substantially simultaneously.

The invention features an arrangement unde the control of the relay in the line termination repeater which receives and repeats signals from the subscriber station, and which will be called the receiving relay hereinafter, for placing the same potential, namely ground, on both transmitting contacts oithe repeater relay which transmits signals to the subscriber station, and

which will hereinafter be called the sending relay, and for delaying the restoration of the normal potentials to the contacts of the last-mentioned relay,-so that if the line termination repeater sending relay seeks to change the potential which it is applying to the lineat an instant that the receiving relay is off-normal, such change in potential will be delayed.

In accordance with the preferred embodiment of the invention, there isadded to the line termination repeater in a system as hereinbefore described, a slow release relay which is energized under the control of the receiving relay each time that relay goes to the off-normal condition, which is the spacing condition. The added relay, upon being energized, substitutes ground potential for a signaling. potential on the spacing contact of the sending relay the substitute ground connection being applied through the operating winding of a second added relay.. When the receiving relay returns to the marking condition, the first added remain operated due to its slow-to-release characteristic. The current through the second added relay will be reversed due to the reversal of battery connection at the subscriber station and will cause the second added relay. to operate to offnormal contact. Through the off-normal contact and armature of the second added relay, the spacing battery connection is restored to the spacing contact of the sending rela'y so that. that re.- lay can retransmit over the line to the subscriber station, the spacing condition-which caused it to operate to its spacing contact. Thus. the retransmission of the spacing signal to the subscriber station following restoration of that'station to the normal or marking condition is delayed only by the interval required. for the second added relay to operate.

The subscriber effects the transmission of a disconnect signal by disconnecting his signaling potential from the line, thus interrupting the line to direct current. The receiving relay in the line termination repeater may be susceptible of operation to spacing as a result of such opening of the line, possibly due to resonance conditions or in the case of utilization of an accelerating vibratory circuit on the receiving relay, thus retransmitting a spacing signal toward the distant subscriber line termination circuit. If at the same. time, the distant subscriber disconnected, the receiving relay in his line termination circuit might operate to spacing and retransmit a spacing signal toward the first subscriber line termination circuit which would cause the sending relay in that circuit to operate to spacing.

By virtue of the provision of the first added relays in the toll line termination circuits, which operate when the receiving relays go to spacing, ground is connected to the spacing contact of the sending relays which prevents recharging of the lines from the spacing batteries and permits the capacity of the line to the subscriber to discharge. The second added relays will not be operated to restore the spacing potential to the spacing contacts of the sending relays because there is no battery connection to the line at the subscriber stations and a suflicient current for operating the second added relays cannot flow in the direction required for operation. Were it not for the provision of these relays for removing the spacing potential from the spacing contacts of the sending relays and substituting ground connection, the subscriber lines might become charged in the opposite direction by the spacing batteries on the spacing contacts of the sending relays, thus restoring the receiving relays to marking and causing a marking signal to be retransmitted toward the remote subscriber toll line termination circuit. The same could ocour in the remote subscriber toll line termination circuit thus setting up an oscillatory condition which might endure indefinitely and prevent the disconnect signal from being registered at the switching station.

. For a complete understanding of the invention, reference may be had to the following detailed description to be interpreted in the light of the accompanying drawings wherein:

Fig. 1 is a diagrammatic view showing a subscriber station circuit;

Fig. 2 is a diagrammatic view showing a subscriber line termination circuit at a central oflice;

Fig. 3 is a diagrammatic view showing a cord circuit for interconnecting subscriber toll lines and the cord circuit includes a repeater;

Fig. 4 is a diagrammatic view showing in simplified forma subscriber line termination circuit and subscriber station circuit identical with the line termination circuit and subscriber station circuit shown in Figs. 2 and 1, respectively;

Fig. 5 is a diagrammatic view showing a modified form of the invention; and

Fig. 6 is a diagrammatic view in block form showing how Figs. 1 to 4, inclusive or 1, 2, 3 and 5 are to be arranged to exhibit complete communication circuits betweentwo subscriber stations according to the two embodiments of the invention shown in the drawings.

The subscriber station circuit shown in Fig. 1 and the cord circuit shown in Fig. 3 are identical with corresponding circuits shown in Patent 2,143,000, granted January 10, 1939, to W. W. Cramer et a1. and the disclosure of this patent is incorporated herein by reference as part of the present specification. The subscriber line termination circuit shown in Fig. 2 represents a modification of the subscriber line termination circuit shown in the Cramer et a1. patent in accordance with the present invention.

Idle condition Reference will now be made to Figs. 1 to 4, inclusive, arranged as in Fig. 6. With the exception of sending relays 22 and 23 and the receiving relay I6 at the subscriber station, the relays in all four figures are shown in their normal positions. These relays remainin these positions during the time the circuit is idle. Relays I6, 22 and 23 at the subscriber station may, however, be on their spacing contacts during the idle condition.

Key 13 is normally in the'position shown in Fig. 1. This key is provided to enable the printing equipment 34 at the subscriber station to be tested locally. The control effected by operation of key I3 is fully disclosed in the Cramer et al. A

patent and will not be repeated here.

At the central ofiice, the relays of the subscriber line termination equipment shown in Fig. 2 and of the cord circuit shown in Fig. 3 are in their normal positions. The term normal position or normal is used to indicate the position assumed by the relays when the circuit is idle. Relays 5| and 52 are maintained on their marking contacts by current flowing in a circuit from ground connected to the back contact of the outer upper armature of relay 58 through the second winding of relay 52, middle winding of relay 5|, inner upper armature. and back contact of relay 58 to grounded battery. Relay 65 is also maintained on its marking contact by current flowing from ground through its lower winding and upper break contact of relay 64 to grounded battery. Relay 6! usually remains on its marking contact as shown in Fig. 2 during idle intervals but need not be so positioned during this time.

Subscriber originates a call The subscriber originates a call by operating the power switch 36 at the subscriber station. The operation of switch 36 completes a path from the source of power 43 through the right-hand pair of contacts of the switch for energizing the printer motor 33, rectifier 24 and relay I4. Relay 14 in operating closes the circuit from line II extending from the central station through recall key I2, test key I3, armature and front consistors, armature and marking contact of relay- 22 on which the armature will now be held due to energization of the right-hand or operating winding of relay 22 from energized rectifier 24,

rectifier 24 from the negative to the positive terminal thereof, marking contact of relay 23 on which the armature of the relay will now be held due to current through the-left-hand or operating winding of relay 23 from rectifier 24, armature of relay 23 to ground. The energized rectifier 24 supplies a source of potential between its terminals and thus in series between ground and line I I extending to the central exchange. Rectifier 24 on becoming energized also causes current to flow through the lower or biasing winding of relay I6 which holds the armature thereof on its marking contact.

At the central station the line I I extends through the armature and back contact of relay 50, upper windings of relays and 52, noise suppression choke coil 54, winding of disconnect relay 55 and upper armature and back contact of relay 56 to ground. Current flowing over this line circuit due to the source of potential applied to the line by rectifier 24 will cause relay 55 to operate. Relay 55, in operating, completes a path from battery on the spacing contact of relay 5| over conductors 25I and 252, front contact and armature of relay 55, conductor 253, winding of relay 5!, conductors 254 and 255, marking contact and armature of relay 5| to ground. Relay 5'! becomes energized over this circuit and completes the energizing circuit of relay 56 which operates. The upper armature of relay 56 ransfers the circuit of line I I from ground on its back relay 52. This transmission circuit is completed in the cord circuit shown in Fig. 3 through the inner upper armature and front contact of relay 84, the back contact and inner upper armature of relay 88, the upper winding of cord circuit repeater relay 90 to the armature of cord circuit repeater relay 9|. About the time of insertion of plug 83 in jack 10, the operator also operates key 89. The upper pair of make contacts of key 89 completes the circuit of relay 88 which operates. The upper armatures of relay 88 connect 4 the operators teletypewriter set comprising recontact to its front contact from which a conductive path is extended through the armature relay 68 through the winding of relay I2 which becomes energized. Relay I2 completes the circuit of call lamp 69 and indicating or alarm device II which indicates to an operator that the subscribers call is waiting to be'answered.

Operator answers The operator answers the call by inserting an answering plug 83 (Fig. 3) in the answering jack 10 (Fig. 2). Thi completes a circuit from ground through the winding of relay 68, sleeve of jack' I6 and plug 63, and winding of relay 84 to battery. Current flowing in this circuit causes the operation of relays 68 and 84. Resistance 246 connectedto grounded battery of the same polarity as that on the winding of relay 84 provides a shunt for the winding of relay 84 to properly control the potential of sleeve of jack 10 and plug 83 and the current flowing through the winding of relays 68 and 84. The operation of relay 68 extends the transmission circuit from the tip of plug 83 through the tip of jack I8, the uppermost from; contact and armature of relay 68, the upper winding of relay 61, the back contact and upper armature of relay 66, the upper winding oflrelay 65 to the armature and marking contact of cording selector magnet 388 and transmitting contacts 389 in series in the communication circuit between the tip of plug 83 and the armature of repeater relay 9|.

Relay 68 in operating completes an obvious circuit through the lower or biasing winding of relay 61. Relay 68 also transfers the ground extended through the lower armature and front contact of relay 56 from the back contact associated with its innermost upper armature to the front contact and the windings of relays 58 and 64. The disconnection of ground from the back contact interrupts the circuit through the winding of relay 12 which releases and in turn extinguishes light 69. Relays 58 and 64 are operated by current flowing in the above circuit through their windings. .Relay 58 in operating disconnects a previously traced path from ground through the second winding of relay 52 and middle winding of relay 5| and connects these windings to the transmission circuit of line II. These windings are now connected between the transmission circuit and the artificial line 53 and it will be noted that the artificial line 53 does not provide any direct current path. No biasing current is required for the receiving relays 5| and 52 because they receive positive and negative signal impulse currents of substantially the same magnitude from the subscriber station. The two lower windings of relay 52 are connected in a vibrator circuit to impart a snap action to relay 52. As soon as the armature of relay 52 is moved out of eng'agement with its marking or spacing contact due to current of the proper polarity in the upper or operating winding, the vibratory circuit associated with the two lower windings quickly and positively drives the armature to the other contact, current for energizing the two lower windings being supplied by a condenser which has previously become charged through the armature of relay 52 from battery on the marking or spacing contact of the relay.

The operation of relay 58 also interrupts the operating circuit of ringing relay 59 and prepares a circuit for theoperation of relay I3 and also connects battery to the lower terminal of condenser 16 through high resistance 18 and the inner upper armature and back contact of relay 263. However, ground is connected to the lower terminal of condenser I6 through the marking contact and armature of relay 6! and the inner upper armature and back contact of relay 263 so that the battery connected through resistance I8 performs no useful function at this time.

The operation of relay 64 connects the lower winding of relay 65 to the transmission circuit through jack 16. This relay also connects a circuit in parallel with the contacts of relay 55 through its lower armature and front contact, a condenser 63 and a resistor in series.

Transmission circuit With key 89 associated with the cord circuit in. Fig. 3 operated, the transmissionpath as above traced isv now complete between the operators printer equipment in the lower part of Fig. 3 and the printer equipment 44 at the subscriber station. Communication signaling impulses may now be transmitted from the operator to the subscriber. With relay 84 operated, battery through the front contact and lower armature is supplied over an obvious path through the left-hand winding of relay 92 causing the relay to operate. The two inner armatures of relay 92 complete conductive paths through the biasing windings of repeater relays 9B and 9| to the transmission path. The outer right-hand armature of relay 92 connects negative battery through the make-beforebreak contacts associated with the inner upper armature of relay 98 and through the makebefore-break contacts associated with the upper armature of relay 95 to the upper winding of relay 9|. The outer left-hand armature of relay 92 seeks to provide a similar battery connection for the upper winding of relay 99 but the path from the armature is open at the inner upper armature of relay 84 which is operated, and the upper winding of repeater relay 90 is receiving the proper polarity through the tip of plug 83. With these circuits for the windings of repeater relays 90 and 9| completed, the armatures are operated to their marking contacts to provide the normal marking current through the transmission path including the operators receiving magnet 388 and transmitting contacts 389. The operator in operating the transmitting contacts 389 of the operators telety-pewriter equipment, interrupts the transmission circuit through jack 19. This provides a spacing condition of no current. This causes the operator's receiving magnet 388 to control the receiving equipment of the operators printer to produce an operator's copy. The relay 65 of the subscriber line termination equipment shown in Fig. 2 also follows the signals, the relay being biased so that it operates to spacing when no current flows through the operating winding, and repeats them through the operating windings of relays 52 and 5| which do not follow signals generated by relay 65, as will be described later and over line to the subscriber station circuit. It should be noted that relay 65 transmits by connecting ground to line H for .marking signal impulses and by connecting positive battery to line H for spacing signal impulses. The positive battery connected through the spacing contact of relay 65 to line H aids the source of negative potential connected to the line H at the subscriber station. The preferred ratio of the two currents for marking and spacing conditions, respectively, is of the order of 1 to 3. To obtain this ratio of current the :potential developed by rectifier 24 is substantially one-half the potential of the source connected to the spacing contact of relay 65. The current flowing through the lower or biasing winding of the receiving relay 6 at the subscriber station in Fig. 1 is adjusted to produce substantially one-half the magnetic effect upon relay It as is produced by the sum of the two currents flowing through the upper winding of relay l6. However, the magnetic effect produced by the current. flowing through the lower winding of relay l6 opposes the magnetic effect of the algebraic sum of the two currents flowing through .the upper windin of relay [6. When these conditions are substantially satisfied, relay l 6 responds equally well to both the marking and spacing signaling conditions. Relay It does not respond to spacing signal generated at the subscriber station because the spacing signal is represented by a reversal of current and the ma netic eflect of the reversed current in the operating winding is to aid the biasing winding in holding the armature of relay I6 on the marking contact.

Relay H5 in responding to the signaling impulses transmitted by the operator repeats them to the printer magnet 32 of the receiving teletypewriter equipment 44 and causes it to record the information transmitted by the operator at the central station. The operating circuit of the printer magnet 32 may be traced from ground through the test key l3, armature and marking contact of relay I6, unilateral conducting device or rectifier 30 in parallel with condenser 3|, windin of printer magnet 32, resistance 45, armature and marking contact of relay 22, rectifier 24, marking contact and armature of relay 23, to ground. Rectifier 24 supplies the current for this path causing the operation of printer magnet 32 in accordance with the signals repeated by relay In th spacing position, receiving relay I6 short-circuits the transmitting contacts 34 and resistance 242. In short-circuiting resistance 242, relay it increases the current through the lefthand Winding of relay 23 and the right-hand winding of relay 22 and thus provides an additional load for rectifier 24 during the time relay I6 is on its spacing contact. This extra load compensates for the load on the rectifier during the time relay I6 is on its marking contacts causing current to flow through the printer magnet 32. In short-circuiting the transmitting contacts 34 in its spacing position, the relay l6 insures the proper reception of a break signal by a transmitting operator at the subscriber station. With the transmitting contacts short-circuited, it is impossible to transmit signals to line H which might interfere with the reception of the break signal or to cause the signal to be recorded by the receiving printer as home copy.

In response to the information transmitted from the central operator, the subscriber will tran mit information regarding the party with which the subscriber wishes to communicate, by

operating the transmitting contacts 34, of the teletypewriter equipment 44. The operation of transmitting contacts 34 opens and closes the circuit through the right-hand winding of relay 22 and left-hand winding of relay 23. Current flowing through these windings is in such a direction as to maintain the relays on their marking contacts. The current flowing through the other windings of these relays is in such a direction as to cause the relays to be operated to and remain on the spacing contacts. However, the current flowing through the right-hand windin of relay 22 and the left-hand winding of relay 23 overpowers the current flowing through the other windings so that as long as this current flows, these relays remain on their marking contacts. When the circuit-of these windings is interrupted by the transmitting contacts 34, the biasing windings of these relays cause the armatures to move to their spacing contacts. In moving to the spacing contacts, the armatures of relaysZZ and 23 reverse the connections of the rectifier 24 to line H whereby a positive potential is now connected between line H and ground instead of negative potential. This causes a spacing impulse to be transmitted over the line H. The armatures of relays 22 and 23 in moving to their spacing contacts also tend to reverse the current flowing through the printer magnet 32 at the subscriber station over a circuit traced from round on the armature and spacing contact of relay 23, rectifier 24 from negative to positive, spacing contact and armature of relay 22, resistance 45, printer magnet 32, unilateral conducting device orrectifier 30, armature and marking contact of receiving relay Hi to ground through the contact of test key l3. The direction of current which would flow through this circuit is opposed by the unilateral conducting device or rectifier 30 so that substantially no current flows through printer magnet 32 at this time, thus causing printer magnet 32 to follow the signal impulses transmitted by transmitting contact ,34 to record a home copy of the information transmitted by the subscriber. However, inasmuch as substantially no current flows through the printer magnet 32, it is necessary to provide a compensating loadcomprising resistances 41 and 48 across the terminals of the rectifier 24 in order to insure substantially constant voltage under both marking and spacing current conditions. With the relays 22 and 23 in the spacing condition, a shunt around resistor 41 comprising the armature and marking contact of relay 22 is removed and a shunt around resistor 48 comprising the arma-.

ture and marking contact of relay 23 is removed. These resistors form substitute load circuit on rectifier 24 when current of reverse polarity is 48 is inserted between the positive terminal of rectifier 24 and ground. Normally the current through the biasing winding of relay l6 tends to cause this relay to move to and remain upon its marking contact. The current normally flowingv through the upper or line winding of relay 6 which is the marking current tends to operate the armature of relay I6 to its spacing contact and maintain it on the spacing contact. However, this current, under marking conditions is insufficient to overpower the biasing current which therefore maintains the relay on its marking contact. When signal impulses are transmitted from the distant station, current through the upper or line winding of relay l6 becomes substantially three times as great as the normal cur rent and thus' overpowers and overcomes the magnetic effect of the biasing current that causes this relay to move to its spacing contact. When spacing impulses are transmitted from the subscriber station, the line current reverses so that the current through the upper or line winding of relay It will tend to hold the relay upon its.

marking contact. Hence the reduction of current through the lower or biasing winding at this time does not permit the relay to leave its marking contact so relay does not follow the signal impulses transmitted from the subscriber station.

The path for current through the lower winding of relay l6 may be traced from ground through the lower winding of the relay and resistance to the negative terminal or rectifier 24. With relays 22 and 23 in the marking condition, the circuit is completed from th positive terminal of the rectifier through the marking contact and armature of relay 23 to ground.

, During the transmission ofspacing current from the subscriber station, ground is connectedto the in Fig. 1.

- release against a low value of leakage resistance,

negativetermlnal of rectifier 24 through the armature and spacing contact of relay 23 thus substantially short-circuiting the lower winding of relay 6 and preventing any current fiow therethrough.

During the transmission of telegraph signal impulses between the subscriber station and the operator and also during the transmission of signaling impulses to or from the subscriber station at any other time, relay 5! (Fig. 2) is maintained operated either from the contacts of relay or by polar relay 5| reversing the charge on condenser 53 through the winding of relay 5?, the charging circuit for condenser 63 being established through the lower armature and front contact of relay 64 which is energized. The function of relay 55 is to register a disconnect condition originated at the subscriber station shown Accordingly relay 55 is arranged to cross-fire and a reasonable value of earth potential when the subscriber removes potential from line H. Relay 55 when so adjusted will be energized on the longer signal pulses and the steady state marking and spacing currents, it being a neutral relay, but may not remain operated during the transmission of telegraph signal impulses. Relay 5| follows signals generated at the subscriberstation. With relay 55 energized and relay 5| on its marking contact, a circuit for relay 5? may be traced from battery on the spacing contact of relay 5| through conductors 25| and 252, armature and front contact of relay 55, conductor 253, winding of relay 5'! and conductors 254 and 255 to the marking contact of relay 5| which is engaged by the grounded armature .of

that relay. When relay 5| goes to spacing relay 5! is energized from ground through the armature and spacing contact of relay 5|, conductors 25i and 252, armature and front contact of relay 55, conductor 253, winding of relay 51, conductor 255, and conductor 24| to battery through the lower armature and front contact of relay 58. Condenser 53 has one terminal connected to the armature of relay 55 through a resistor and has the other terminal connected to the front contact of relay 55 through conductor 25B and 252, From this it will be apparent that when relay 55 is energized, it short-circuits the condenser 63 but that when relay .55 releases, which it may do when rapid telegraph signals are being transmitted from the subscriber station and which it should do when the subscriber disconnects, the

, condenser is inserted in the energizing circuit of relay 5'! between ground through the armature and marking contact of relay 5| and battery on the spacing contact when relay 5| is in the marking condition or between ground through the armature and spacing contact of relay 5| and battery on the lower armature of relay 68 when relay 5| is in the spacing condition. Thus as the armature of relay 5| operates between its marking and spacing contacts in following signals, it reverses the polarity on condenser 53 whichwdischarges and recharges through the winding of relay 5?. Relay 5! is an alternating current relay and remains operated in response to the discharging and recharging currents of condenser 63 and also holds energized from steady currents of either polarity flowing through the armature and front contact of relay 55, when that relay is energized. Thus, even though relay 55 may release in response to rapid telegraph signals, relay bringing in of a disconnect indication or other disturbance of the activated condition of the subscriber line termination circuit.

- Relay 56 which is operated by relay 51 and when operated extends the communication path of lin I! through to the outer upper armature of jack relay 68, is a slow-release relay which further serves to protect the circuit arrangement from false supervisory signal indications at the central switchboard due to any momentary releases of relay 51 which might occur between cessation of charging current for condenser 63 and reenergization of relay 51 through the armature and front contact of relay 55.

Relay I is provided with a third winding in addition to the two windings similar to the windings of relay 52. The third or lower winding 'is connected to the contacts of relay 5I in such a manner that an additional or biasing current flows through this winding in such direction as to maintain relay 5| on the contact on which it is resting. In other words this winding tends to make relay 5| less sensitive than relay-52. This is desirable in order to prevent false operation of relay 5| on cross-fire currents and other interference received during the time the line II is open-eircuited at the subscriber station or has no potential connected to it at the subscriber station.

Completion of connection to called subscriber After the operator at the central station has received all the necessary information from the subscriber she can connect the subscriber to the desired station by inserting the calling plug IOI of the cord circuit into the jack I39 of a called subscriber line termination circuit. This circuit together with the subscribers station circuit are shown in Fig. 4. They may be identical with the corresponding circuits shown in Figs. 1 and 2 but they have been shown in simplified form. Corresponding elements have been identified by the same reference numerals in Fig. 4 as in Figs. 1 and 2 with the exception of identification of the line termination circuit jack by the reference numeral I39 which is intended to indicate that this is a different subscriber's line termination cir-.

cuit. The operator will then operate ringing key I96 which causes ringing current to be transmitted through the tip of plug IOI and over the communication path through the subscriber line termination circuit established by the operation of relays in that circuit as a result of the insertion of the plug I9I into jack I39 and over the line to the called subscribers ringer 40. The called subscriber answers by operating key 36 which completes the circuit of relay I4 to extend the communication path through to ground on the armature of sending relay 23 and which also completes the circuit of the source of power 43 to rectifier 24 and to operating motor 33. A communication path has now been established from the cord circuit to each of the subscriber stations that are tobe interconnected. The operator may then disconnect the recording and transmitting apparatus at the central office by opening key 89. This releases relay 88 which discontact.

battery through the front contact and lower armature of relay 98 which became energized upon insertion of plug ID'I into jack I39 so that relay 92 remains energized to maintain the proper potentials on the biasing windings of repeater relays 90 and 9| and for other purposes.

Break signals engagement with its marking contact, the winding of relay 268 is short-circuited. When the armature of relay 52 goes to the spacing contact, an energizing circuit for relay 268 may be traced from battery of one polarity on the marking contact of relay 52 through resistor 266, conductor 261, winding of relay 268, conductor '269, resistor 21 I, armature and spacing contact of relay 52 to battery of the opposite polarity. Since relay 268 is slow to release, it does not follow rapid oscillation of the armature of relay 52 involved in the retransmission of signals received from the subscriber station of Fig. 1 but remains energized and holds its armatures. Whenthe armature of relay 52 remains-on its marking contact for an interval longer than the release time of relay 268, the latter relay releases its armatures.

The lower armature of relay 268 is connected by conductor 212 to the spacing contact of sending relay and the back contact for the lower armature of relay 268 is connected to battery 213. This battery becomes connected to the line II extending to'the subscribers station of Fi 1 when relay 65 goes to its spacing condition with relay 268 released and as previously stated, battery 213 is in series-aiding relation to the marking signal potential at the subscribers station and has twice the voltage of that potential so that a spacingsignal retransmitted by relay 65 is represented by current in the same direction and'of substantially three times the magnitude of the marking current which flows when th armature of relay 65 is on its grounded marking The front contact associated with the lower armature of relay 268 is connected to one terminal of the operating winding of a biased polar relay 214, the other terminal of which is connected to the armature of that relay and through resistor 216 to the front contact associated with the outer upper armature of relay 268. The outer upper armature of relay 268 is connected to battery 213 as is also the inner upper armature. The biasing winding of relay 214 has one terminal connected to ground and the other terminal connected through resistor 211 and through resistor 218 to battery 213. The inner upper armature and back contact of relay 268 establishes a direct shunt around resistor 218 when relay 268 is released and the biasing current which flows from battery 213 through the biasing winding of relay 214 is in a direction to drive the armature of relays 214 to it grounded left-hand contact. There is no energizing circuit for the operating winding of relay 214 when relay 266 is released because the circuit of that winding is open at the lower front contact of relay 268. When relay 268 becomes energized, the shunt around resistor 218 is removed and this resistor reduces the current flowing from bat tery v213 through the biasing winding of relay 214. With relay 263 energized the circuit of the operating winding of relay 214 is extended over conductor 212 to the spacing contact of relay 65, the armature of which remain on its marking contact except when signals are to be retransmitted to the subscriber station of Fig. 1. From this it follows that even though relay 268 becomes operated and remains operated during transmission of telegraph signals originating at the subscriber station shown in Fig. 1, the circuit of the operating winding of relay 214 will not be completed unless the relay 65 is operated to its spacing contact to retransmit-a signal, such as a break, to the subscriber station of Fig. 1. It will be noted that with relay 268 operated, the path from the spacing contact of relay 65 is to ground through th operating winding and armature of relay 214, which is the same potential as that on the marking contact of relay 65, so that no change in the current flowing in the line II will result immediately upon the-engagement of the armature of relay 65 with its spacin contact. However, with the operating Winding of relay 214 included in the circuit of line II when relay 65 goes to its spacing contact,

the operating winding of relay 214 becomes en-' ergized and overpowers the biasing winding if marking potential is applied to the line at the subscriber station, driving the armature away from the left-hand contact. As soon as the armature leaves the left-hand contact, the ground connection is removed from the armature but a conductive path is thentraced from the spacing contact of relay 65 through conductor 212, the lower armature and front contact of relay 258, operating winding of relay 214, resistor 216 and front contact and outer upper armature of relay 268 to battery 213. This results in a maintenance of the current through the operating winding of relay 214 to insure that the armature will travel all the way to the marking contact. The operating winding f relay 214 remains energized over this circuit and completes the movement of its armature to the right-hand contact. When the armature reaches the right-hand contact which is connected to battery 213, it establishes a shunt around resistor 216 thereby causing the I the transmitted signals, desires to transmit a break signal to interrupt transmission from the station in Fig. 1.

The initiation'of the break signal isaccomplished by operation of key I59 at the subscriber station. This key interrupts the circuit of the operating windings of relays 22 and 23 thereby reversing the polarity at the subscriber station. If the sending relay in the line termination circuit at the left of Fig. 4 should be in the spacing condition in response to a spacing signal at the instant that the break is initiated, relay 52 will not be operated to spacing in response to the break signal of spacing nature until relay 65 returns to marking because although the spacing battery 213 supplied through the back contact and lower armature of relay 268, spacing contact and armature of relay 65 and upper winding of relay 52 is opposed by the spacin polarity of the breaksignal at the subscriber station, the battery 213 is substantially of twice the voltage of the source at the subscribers station and current corresponding to the normal marking current flows in the line circuit between the subscriber circuit and the line termination circuit under these circumstances. If the relay 65 is in the marking condition or when it returns to the marking condition, relay 52 operates to spacing transmitting a spacing signal through jack I39 and plug Nil into the cord circuit repeater of Fig. 3.

With relay 52 in Fig. 4 operated to spacing, relay 268 operates, removes the shunt from around resistor 218 in the circuit of the biasing winding of relay 214, thereby reducing the biasing current, disconnecting battery 213 from the spacing contact of relay 65, and substituting therefor ground through the armature and operating winding of relay 214 from the right-hand contact of relay 214. Thus if the relay 65 should again be operated to spacing after the break signal has been extendedinto the cord circuit, the current through the operating winding of rereceiving relay 52 cannot be reversed, so that relay 52 will remain in the spacing condition. Current which would flow through the operating winding of relay 214 under this circumstance would supplement the biasing winding in holding the armature on the grounded contact so that the relay 214 will not operate to restore the spacingbattery 213 to the spacing contact of relay 65.

Repeater relay 9! in Fig. 3 responds to the spacing signal transmitted by relay 52 of Fig. 4 and operates to spacing irrespective of the con dition of repeater relay 90, whether marking or spacing.- The reason for this is that there is no I current through the operating winding of relay does not occur when the subscriber station of Fig.

transmittingand that the subscriber at the station' atthe right of Fig-4, which is receiving;

9| when relay is marking and relay 52 is spaging, batteries of like polarity being in opposition, and relay 90 biases relay 9| to spacing whereas when relays 9D and 52 are both spacing, two spacing batteries of opposite polarity are in series-aidingrelation to cause current to flow in the opposite direction. from that produced by series-aidingmarking battery and although relay 90 would be biasing relayfll to marking, the current produced by the series-aiding batteries on the spacing contacts of relays Stand 52 would overpower the biasing winding of relay 9| and drive the armature to spacing. Thus the spacing signal is transmitted out of the cord circuit throughplug' 83 and into the subscribers line termination of Fig. 2 through jack 10.

-;:.Sending.relay 65 in Fig. 2 responds to the spacing signal transmitted by relay 9| in Fig. 3 and operates to spacing irrespective of whether receiving relay 52 is in the marking or spacing condition, for the same reasons as those set forth with reference to the operation of relay 9|, because similar conditions of marking and spacing polarities and similar biasing conditions exist.

If relay 52 is in the marking condition and has been in that condition for a sufiicient interval of time to permit relay 268 to release, the armature of relay 85 finds spacing polarity on its spacing contact from battery 213 and at once transmits the spacing signal toward the subscriber's station of Fig. 1. If relay 52 is in the spacing condition, relay 268 will be in the operated condition and the potential will be ground on the spacing contact of relay 214, supplied through the armature and operating winding of relay 214 and the front contact and lower armature of relay 268. The relay 214 will not be energized because'the spacing condition of relay 52 indicates a positive polarity coming in on line H which is not the polarity re-: quired by relay 214 to operate its armature. However, when the line II is restored to marking, relay 214 operates its armature to the right-hand contact, thereupon connecting the spacing battery 213 to the spacing contact of relay65 through the operating winding of relay 214 and efiecting the transmission of the spacing signal to thesubscribers station in Fig. 1.' When relay 52 has remained on its marking contact for a sufiicient interval for relay 268 to become deenergized, that relay releases, interrupting the circuit through the operating winding of relay 214 and connecting spacing battery 213 directly to the spacing contact of relay 65.

In response to the break signal of spacing nature, relay l 6 at the subscribers station inFig. 1 goes to spacing, interrupting the circuit through the printer controlling selector magnet 32 and short-circuiting the transmitting contacts 34 so that they will be ineffective to operate transmitting relays'22 and 23. At the time that the sending relay 55 in Fig. 2 responds to the break signal, relay 81 also responds to that signal and sets in operation a timing circuit including electron discharge tube 15. The operations controlled by relay B1 are fully described in the Cramer et al. patent hereinbefore identified and the description will not be repeated in the present specification.

Disconnect At the termination of transmission between the subscribers connected together through the central exchange, the subscribers stop their machines by releasing the power switches 36. This disconnects power from the teletypewriter motors 33, rectifiers 34 and relays I4. Relay l4 releases and connects the line extending from the subscriber line termination circuit to ground through ringer 40-and condenser 39. This removes all sources of potential from the line at the subscriber station. In the subscriber linetermina'tion circuit, this 111- terrupts the conductive path through the winding of relay 55 which should release. Relay 55 when it does release interrupts the direct current energizing circuit for relay 51 and if relay 5| remains on one or the other of its contacts so that condenser 63 does not discharge and recharge 'in opposite directions, relay '51 will be released, thus releasing the relay 56. Relay 56 releases relays 64 and 58 and establishes a circuit through its lower armature and backcontact and through the lower armature and back contact of relay 262 to the ring contact spring of jack and through thering of plug 831to relay 85 which atits outer lower armature and front contact completes a circuit for lamp 86 to inform the operator of the disconnect and to indicate to her that plug 83 should be removed from jack 18.

However, relay 55 may fail to release or relay 51 may fail to remain on one of its contacts or both of these failures may occur under circumstances now to be described and it is for the purpose of guarding against such failure that the system comprising relays 268 and 214 have been provided. The circumstances under which relay, 55 may fail to release or relay 51 may fail to remain On one contact will first be described as if the relay system comprising relays 268 and 214 were not provided and under these circumstances the spacing battery 213 for the transmitting relay 65 would be connected directly to the spacing contact of that relay.

With the line from the subscriber station to the line termination circuit opened to direct current at the subscriber station, the line capacity will discharge through'the winding of receiving relay 52 and cause the armature of that relay to hold, but oscillation may occur due to resonance between the line and the repeater elements of the line termination circuit and such oscillation may cause the armature of relay 52 to go from marking to spacing even if the relay has no vibratory circuit. With a vibratory circuit associated with the third and fourth windings of the relay such circuit will amume control of the armature and oscillate it between its marking and spacing contacts. Thus, although the disconnect has been effected with the line H between the subscriber station and the line termination circuit in the marking condition, the receiving relay is driven to the spacing contact and a spacing signal will be transmitted toward the other subscriber station shown in Fig. 4. The subscriber at the station shown in Fig. 4 may have disconnected at about the same time, similarly causing the receiving relay 52 in the line termination circuit to operate to spacing. Thus these two spacing signals are transmitted in opposite directions and each causes the sending relay 65 to which it has been transmitted to operate to spacing. As previously mentioned, the sending relay 55 responds to and retransmits received signals independently of the condition of the receiving relay 52. Accordingly, battery 213, assumed to be connected directly to the spacing contact of relay 65 in each of the line termination circuits, would be connected to the line through the spacing contact and armature of relay 65 and through the winding of receiving relay 52, charging the line through the operating winding of the receiving relay 52 and causing the armature of the receiving relay to return to its markin-g contact whetheror not that relay had a vibratory circuit. This would occur in both of the subscribers line termination circuits. Upon the return of the receiving relays 52 to marking, they would transmit a marking signal in opposite directions through the cord circuit repeater to cause both of the sending relays 55 receiving such marking signals to return to marking whereupon the line extending from each subscribers station to its line termination circuit could again discharge to ground through the armature and marking contact of the sending relay thus causing the receiving relay 52 again to operate to spacing and start another cycle like the one just described. In this way an oscillatory condition could be set up which would continue indefinitely. Since in each of the subscriber line termination circuits the relay is connected in series with the operating winding of relay 52, the currents traveling first in one direction and then in the other as a result of the oscillatory condition would cause relay 5| to follow the oscillations which would maintain the relay 51 energized thus preventing the release of relays 56, and 64 and the operation of the lamp 86 associated with the cord circuit, and the operator would receive no indication that plugs 83 and IBI should be removed from jacks 1|] and I39, respectively.

Oscillation between the two line termination circuits may develop even though neither has a vibratory circuit on its receiving relay and even though only onedevelops oscillation between its repeater elements and its subscriber line of sufficient amplitude to operate its receiving relay to spacing as a direct result of the disconnect on its line. Assume that both subscribers disconnect about the same time, with their lines in marking condition. The lines will be charged negatively at the time of disconnect, and the discharge of the negative charge through the upper windings of relays 5| and 52 in both line termination circuits, and through the armaturesand marking contacts of the sending relays 65 to ground, will be in the direction to hold the relays 5| and 52 on their marking contacts. Assumefurther that oscillation occurs in one of the line termination circuits which causes its relays 5| and 52 to operate to spacing. This will cause a spacing signal to be transmitted toward the other line termination circuit Where the relays 5| and 52 will be assumed to have remained on their marking contacts, and where the sending relay 65 will operate to spacing in response to thespacing signal, thus connecting positive potential to its subscriber line and recharging the capacity of that line. This recharging current is in the direction to hold the relays 5| and 52 on their marking contacts. However, the surge of charging current from the positive spacing battery may overcharge the line, which will be followed by a partial discharge of the capacity charge, which is positive. The current through the operating windings of relays 5| and 52 resulting from this partial discharge is in the direction to drive these relays to spacing, and thus a spacing signal may be transmitted toward the remote line termination circuit, operating the sending relay thereat to spacing and charging the capacity of the line positively, restoring the receiving relay to marking. One line termination circuit has thus been drawn into participation in oscillation by the other and such oscillation may continue indefinitely.

The manner in which the circuit comprising relays 268 and 214 prevents the occurrence of oscillation, between the line termination circuits will now be described. As previously stated, the receiving relay 52 is susceptible of operation to spacing when the subscriber disconnects and accordingly if oscillations due to resonance occur in the circuit, which operate relay 52, they will also operate relay 5| and prevent the registration of the disconnect signal. Assume first that both relays 52 operate to spacing due to local oscillation. This may occur in both of the subscriber line termination circuitsor only in one. As a result of the operation of relay 52 to spacing, slow-release relay 268 is energized, disconnecting spacing battery 213 from the spacing contact of sending relay 65 and substituting a ground connection through the operating winding and armature of relay 214 to the groundedcontact of that armature which the armature normally engages by virtue of the biasing circuit. When the sending relay 65 receives the spacing signal retransmitted by the receiving relay at the remote subscriber line termination circuit, the armature of the sending relay operates to the spacing contact where it finds ground connection through the operating winding of relay 214, corresponding to the connection on its marking contact, instead of finding spacing battery connection, The line continues to discharge to this ground connection and by virtue of the removal of spacing battery 213, no potential is applied to the line extending toward the subscriber station through the operating winding of receiving relay 52 to cause it to return to marking condition. The potential stored in line II will be polarized to operate the armature of relay 214 toward its grounded contact so that relay 214 will remain unoperated and spacing battery 213 will not become connected to the spacing contact of sending relay 65 through the winding of relay 214 as it does in the case of a break signal.

Upon the discharging of the line to ground, relay 51 will release and through the chain of operations previously described, will cause the lamp B6 to be lighted to inform the operator of the disconnect. Thus the oscillatory condition is prevented and the establishment of the disconnect indication is assured. The end of the cord circuit shown in Fig. 3 which terminates at plug IIlI has a lamp I02 controlled similar to the lamp 66 through the ring circuit of the plug IIlI and jack I39 forindicating the disconnect originated at the subscriber station shown in Fig. 4.

- Assume now that the subscribers disconnect with their lines in marking condition and that only one termination circuit undergoes local oscillation of sufiicient amplitude to drive its relay 52 t spacing. As previously stated, the discharge of the negative charge through the upper windings of relays 5| and 52 and the marking contact of relay 65 to ground is in the direction to hold relays 5| and 52 to their marking contacts, and at the, termination circuit which undergoes only low amplitude oscillation or none, relays 5| and 52 will be held on marking and relay 268 will remain deenergized. However, the sending relay may receive a spacing signal from the receiving relay at the remote subscriber line termination circuit. When the sending relay 65 operates to its spacing contactypositive potential will be applied to the line through the spacing contact of relay 65 and the upper windings of relays 52 and 5| because relay 268 is unoperated, and the positive potential will charge the line capacity, This current is in a direction to hold the relays 52 and 5| on their marking contacts. If the line capacity overcharges, then partially discharges, and drives relays 5| and 52 to spacing, or when relay 65 receives a marking signal from-the remote subscriber line termination and its armature returns to the marking contact, the line capacity discharge through'the upper windings of relays 5| and 52 and the marking contact of relay 65 to ground will cause relays 5| and 52 to operate to spacing, and relay 268 willbe operated to remove positive battery connection from the spacing contact of relay 65. The discharge will be in the di- "rection to hold the armature of relay 214 on its grounded contact and the line will be enabled to discharge fully to ground. At the remote line termination circuit the relay 268 will have been operated incident to the transmission of the spacing signal by its receiving relay. and its line cannot become recharged when a spacing signal is returned to it. Thus oscillation between the termination circuits will be suppressed.

Modified disconnect oscillation suppressing circuit Fig. shows a modified form of disconnect oscillation suppressing circuit for use in a twowire trunk circuit interconnecting a concentrating unit switching center and a central ofiice. The modified disconnect oscillation suppressing circuit provides for a longer interval for discharging a line before the polar relay which is operable to restore spacing potential to the spacing contact of the sending relay becomes connected to the line than does the oscillation suppressing circuit shown in Figs. 2 and 4.

In Fig. 5 the fundamental elements of the central office termination and concentrating unit termination of a two-wire trunk circuit and of a subscriber station circuit served by the concentrating unit are shown. These circuits are fully and completely disclosed in copending application Serial No. 462,966 filed October 22, 1942, by W.

\ V. K. Large et a1. and the modified disconnect oscillation suppressing circuit associated with the central ofi'ice termination of the trunk in Fig. 5 is shown but not claimed in said copending application. The disclosure of the copending application is incorporated herein by reference as part of the present specification.

The trunk termination circuit at the central ofllce, shown at the left o'f'Fig. 5, has a "jack 539 adapted to receive plug I!!! .of the cord circuit shown in Fig. 3, through which the trunk termination circuit maybe connected to the subscriber line termination circuit of Fig. 2 and through it to the subscriber station :of Fig. '1. The tip of marking contact of sending relay 565 is ground-' ed and with all of the relays in the marking' condition the line wire 5! and neutralizing wire 5!2 have ground connection at the central ofilce trunk termination.

At the concentrating'unit, line 5! is connected to an armature of relay 5!6, the front contact of which is connected through the first winding of receiving relay 52! to the armature of sending relay 522 which has negative battery connection on its marking contact and positive battery connection on its spacing contact. The armature of sending relay 522 is also connected through the second winding of relay 52!, which is a biasing winding, and through an artificial line to ground. The neutralizing wire 5!2 is connected to an armature of relay 5l6, the front contact of which is connected through the fourth winding of m jack 539 is connected through the operating wind- I ing of a biased polar relay 564 and through the operating winding of sending .relay 565 to the armature of receiving relay 1552. The relays 565 and 552 in the trunk termination at the central ofiice correspond. to the relays '65 and 53 in the subscriber line termination circuit in Fig. 4. The

biasing windings of relays 5'64 and 565 are connected in series to the armature of receivingrelay 552. The marking and spacing contacts of relay 552 have negative and positive battery connections respectively. The armature of sending relay 565 is connected through one winding of disconnect relay 555 which corresponds to disconnect relay 55 in the subscriber line termination of Fig. 4 and through the uppermost winding of receiving relay 552 to line 5! l which extends to the concentrating unit. There is a .connection from the first winding of relay 55.2 to one terminal of the second winding, the other terminal of which is connected to an artificial line which provides no conductive path for direct current, .so that relay 552 is'not'biased by the second winding. The third and fourth windings of relay 552 have a vibratory circuit associated therewith connected .to the armature of relay 552. The armature of relay 564 is grounded and :from the marking con.- tact of that relay a path is traced through the second winding of central oflice trunk (115C011?- nect relay 555 and the fifth winding of receiving relay 552 to .a neutralizing wire 5!.2 which .extends to the concentrating unit. There is a-connection from the fifth winding of relay 552 to one terminal of the sixth winding, the other terminal of which is connected .to .an .artificial line which provides no direct current path so that relay 552 is not biased by the sixth winding, The

ceiving relay 52! and through a resistor 523 to ground. There is also a connection from ground through the third winding of relay 52! which is a biasing winding and through an artificial line to ground. The fifth winding of relay 52! has a battery connection on one of its terminals and ground connection on its other terminal and provides-a steady bias for relay 52 l The armature of receiving relay 52! is connected through the operating windings of sending relay 522 and break relay 524 and the winding of concentration unit disconnect relay 52'! to line 526 which extends to the subscriber station circuit shown at the right of Fig. 5. The biasing windings of sending relay 522, and break relay 524 are connected in series between the armature of relay 52! and ground. The marking contact of receiving relay 52! has positive battery connection and the spacing contact has ground connection supplied through the armature and marking contact of break relay 524. The spacing contact of the break relay is connected to the marking contact of receiving relay 52! so that if receiving relay 52! should go to the spacing condition while relay 524 is in the spacing condition, marking potential will be applied to the spacing contact of relay 52! andrelay 52! will be unable to change the potential applied 'to line 526 at the concentrating unit.

and line 526 is shown extending directly from the sending and break relays through the discon nect relay to the subscriber station and specifically to the swinger spring of a key 53! through which connection is made in the idle condition of the station circuit to ground'through a ringer 532 and condenser 533 for passing alternating ringing current only. When the subscriber station circuit is to be connected to line 526 key 5.3! is operated to its lower contact to extend a conductive path' from line 526 through the operating winding of line relay 534, break key 536 which is shunted by resistor 535 and transmitting contacts 53*! to ground. The marking contact or line relay 534 is grounded and the armature is connected through teletypewriter selector magnet 538 to grounded battery. The lowerwinding of line relay 534 biases the armature to spacing but when ,key 53! i operated to its lower contact and current flows over line 526 from the marking contact of sending relay 52! current'in theoperating winding of relay 534 overpowersthe biasing winding and completes the energizing circuit of selector v magnet 538. The effect of operation of transmitting contacts 531 is to open and close the circuit of relays 522, 524, 521 and line relay 534 at the subscriber station thus operating relays 522, 524 and 534 to spacing, and the sending relay 522 repeats the spacing condition over line l to the receiving relay 552 in the trunk termination at'the central oflice. Relay 521 is shown slow release and does not release its armature in response to norrnal spacing signals. Relay 521 does not release when break key 536 is operated, because resistor 535 passes sufficient current to hold relay 521 operated although not enough to prevent operation of relay 522, 524 and 534 to spacing. In the system shown in the copending application of Large et al., the relay corresponding to relay 521 is not slow release, but it controls the release of other relays, one of which is slow release and controls the release of the relay corresponding to relay 5 l 6. For the sake of simplicity, the intermediate relays have not been shown, and the delayin the release of relay 5| 6 is indicated by showing relay 521 as being slow release. When relay 521 receives signals from the central ofiice to be repeated to the subscriber station its armature operates between the marking and spacing contacts and when on the spacing contact line 526 is grounded at both ends and the armature of line relay 534 at the subscriber station operates to spacing thereby deenergizing selector magnet 538, which is the spacing condition. The operation of receiving relay 552 in the trunk termination circuit at the central office under the control of sending relay 522 at the trunk termination circuit in the concentrating unit is the same as the operation of the receiving relay 52 in Figs. 2 and 4, under the control of transmitting relays 22 and 23, since sending relay 522 reverses the polarity applied to line 5 in the same manner that the sending relays at the subscribers station shown in Figs. 1 and 4 reverse the polarity. on their respective lines.

The armature of receiving relay 552 when on the marking contact short-circuits the winding of a relay 568 which corresponds to the relay 268 in Figs. 2 and 4 and has its lower armature connected to the spacing contact of sending relay 565. The back contact with which the lower armature cooperates is connected to spacing battery 513 which provides substantially twice the voltage available at the sending relay 522 and is in series aiding relation to the voltage applied to the armature of relay 522 when the relay is in the marking condition and sendin relay 565 is in the spacing condition with battery 513 connected to the spacing contact directly, when relay 568 is released, or indirectly, as will be described later. When signals are being retransmitted at a normal rate by relay 552 the relay 568, being slow release, holds its armature. The

lower armature then engages a front contact which is connected to the lower armature of a relay 516 for which there is no corresponding relay in Figs. 2 and 4. The grounded inner upper armature of relay 568 cooperates with a back contact connected to one terminal of the winding of relay 516 which has a slow-release characteristic,

. is connected to the upper winding of biased polar relay 514, the other terminal of which is connected to the armature of relay 514 which normally engages its grounded right-hand contact. Thus a path is traced from the spacing contact of relay 565 through the lower armature and front contact of relay568, lower armature and back contact of relay 510 and upper winding and armature of relay 514 to the grounded contact which the armature engages. The armature of relay 514 i urged into engagement with the grounded contact due to current in its lower winding, which is the biasing winding, from battery 513 through resistors 518 and 511 and the lower winding of relay 514 to ground. With relay 552 transmitting signals into the cord circuit relay 565 is or should be in the marking condition so that a circuit through the upper winding of relay 514 is not completed. The middle winding of relay 514 has one terminal connected to ground and the other terminal connected to the marking contact of re lay 564 which should also be in the marking condition. Thus only the lowermost or biasing winding of relay 514 will be energized.

If the relay 552 should be transmitting a spacing signal at the instant that relay 565 responds to a break signal of spacing nature from the cord circuit, the current through line 5| l, upper winding of relay 552, right-hand winding of relay 555, armature and spacing contact of relay 565, lower armature and front contact of relay 568, lower armature and back contact of relay 510 and upper winding and armature of relay 514 to ground will be in the direction to aid the biasing winding of relay 514 and this relay will not be operated. The spacing signal of the break will not be retransmitted by relay 565 because the spacing contact is receiving the same potential as the mark ing contact, namely, ground through the armature and upper winding of relay 514. With relay 564 operated to spacing along with relay 565 a short circuit is removed from around the middle winding of relay 514 and this Winding becomes connected in series with the neutralizing wire 5l2. The circuit of the neutralizing wire is extended through a winding of relay 514 because the circuit of line 5 has also been extended through a winding of relay 514 and it is desired that both circuits shall influence the operation of rela 514 in the same manner that they influence the operation of relays 552 and 521, since both circuits include windings of these two relays. However, the middle winding of relay 514 will not cause movement 'of the armature to the lefthand contact since the upper winding is aiding the biasing winding in holding the armature on the right-hand contact.

When the relay 522 responds to and repeats a marking signal toward the trunk termination at the central office, the polarity on the upper winding of relay 514 will be reversed and the armature will be operated out of engagement with its grounded right-hand contact. As soon as the armature leaves the grounded contact a substitute circuit for causing the armature to continue to move to its left-hand contact becomes eifective, the circuit being from grounded battery 513 through the front contact and outer upper armature of relay 568 which has not yet released its current through the upper winding of .relay 514 and over line conductor to the full spacing value- This .causes the relay 52%! to operate to spacing and the break signal is repeated to the subscriber station at the right of Fig. 5. With the break signal of spacing nature repeated to .the subscriber station, relay 522 remains in the marking condition and relay 552 also remains in th marking condition. Relay 568 releases its armatures after its delay interval, thus reenergizing relay 516, reconnecting battery 513 to the spacing contact of relay .565 independently of the upper winding of relay 514 and opening the path frombattery 513 through resistor 516 to the upper Winding of relay 5T4. Relay 518 in becoming energized prepares a path to the spacing contact of relay 565 from ground on its lower front contact through its lower armature to the lower front contact of relay 568, and connects ground directly to. the neutralizing wire by means of the grounded front contact and inner upper armature of relay 510, which ground serves as the terminal point of neutralizing wire 5!2 independently of the ground connection at the marking contact of relay 564, which becomes disconnected from the neutralizing wireeach time relay 584 responds to a spacing signal from the cord circuit of Fig. 3 in unison with relay 565. At its outer upper armature'and front contact relay 518 places a shunt around resistor 518 to increase the bias-.

ing current in the lower winding of relay 514 and thus to reduce the sensitivity of that relay. It will be noted that the normal condition of the relay circuit comprising relays 568, 516 and 514 during retransmission of signals from the cord circuit of Fig. 3 and toward the subscriber station circuit of Fig. 5 by relay 565 is with relays 568 and 514 unoperated and relay 510 operated. It will also be noted that the retransmission of the break signal of spacing nature to the sub- 4 scriber station of Fig. 5 has been delayed only by the interval required for relay 514 to operate, which is comparable with the operation of the relay system comprisingr'elays 268 and 214 in Figs. 2 and 4 and the addition of relay 518 has not changed this negligible delay.

A disconnect is effected at the subscriber station in Fig. 5 by operating the swinger of key 53! to its upper contact which connects the ringer and condenser in series with line 526 thus opening the line to direct current. This causes relay 522 to operate to spacing and to transmit a spacing signal over line 5!! which drives. relay 552 to spacing positively and with or without the presence of the vibratory circuit on its third and fourth windings. With line 526 open to direct current at the subscribers station circuit relay 521 releases and in turn releases relay 5!6. This relay disconnects trunk conductor 5!! and neutralizing conductor 5!2 from the first and fourth windings of receiving relay 52! at the concentrating unit trunk termination and forms a loop across these conductors at the concentrating unit through relatively high resistance 5!1 in shunt with alternating current relay 5-!8 and condenser 5!!]. Relay 5! 8 is an alternating current relay and it is operable in response to ringing current transmitted from the central oflice to initiate operations which will result in the energization of relay 5!6 when the trunk is seized at the central office for'establishing a communication circuit through to a subscriber station served by the concentrating unit, asis fully described in the copending application of Large et .al.

With relay 5! 6 released there is no source of current connected to line 5!! at the concentrating unit trunk termination but due to the presence of the vibratory circuit on the third and fourth windings of relay 552 at the central office trunk termination, the armature of that relay will be vibrated between its marking and spacing con-' tacts. In the absence of a vibratory circuit the armature of relay 552 would remain on the spacing contact to which it has been driven by the operation of relay 522 to spacing before relay 521 released relay 5!6 and disconnected line 5!! from relay 522. Assuming that the communication system has been in the marking condition at the time the disconnect is effected relays 568 and 514 will be unoperated and relay 510 will be energized. With relay 552 operated to spacing, relay 568 is Operated and the spacin battery 513 is disconnected from the spacing contact of relay565 and a ground connection is substituted through the lower armature and grounded front contact of relay 510. The energizing circuit of relay 510 is interrupted as a result of the energization of relay '568 but the relay 510 is slow to release and thus the spacing contact of relay 565 is not immediately connected to an operating winding of relay 514 upon the operation of relay 568 as is the case of the previously described system shown in Figs. '2 and 4 wherein the spacing contact of 'relay 65 becomes connected to an operating winding of relay 214 as soon as relay 268 operates as'the result of operation of relay 52 to spacing in consequence of the disconnect operation at the subscribers station. When relay 518 releases after an interval the spacing contact of relay 565 does become connected to the upper winding of relay 514, the short circuit around the middle winding of relay 514 is removed and the sensitivity of relay 514 is increased due to the removal of the shunt around resistor 518. Should the relay 565 respond to a spacing signal resulting from operation of relay 52 in Fig. '2 to spacing in consequence of initiation of a disconnect at the subscriber station in Fig. 1 any time before the release of relay 510, the interconnected line 5!! and balancing wire 5!2 will not be recharged, the windings of disconnect relay 555 and the first and fifth windings of relay 5! will not be reenergized, because spacing battery 513 is disconnected from the spacing contact of relay 565 and that contact is grounded at the lower armature and front contact of relay 510 so that the line 5! I and neutralizing wire 5 !2 are afforded ample opportunity to become completely discharged. After the release of relay 510 the spacing contact of relay 565 still has ground connection supplied through the upper winding of relay 514 and its armature and left-hand contact so that the line 5! and neutralizing wire 5! 2, if not already fully discharged, will continue to discharge and will not be recharged. By virtue of the addition of slow-release relay 510 to the system a longer interval of time is afforded for discharging line 5! and neutralizing wire 5!2 before the lines become connected to the operating windings of relay 51 4,and the release of disconnect relay 555 is assured. Thus if it should happen that at the time the relay 568 is'operated as a result of the disconnect the residual charge on line 5!! and neutralizing wire 5!2 is of sufficient potential and of the proper polarity to operate relay 514, that relay will not be operated because its operating windings do not become connected to the line 5! I and neutralizing wire 5!2 until relay 516 has released and the relay 511] affords ample time for dissipating such residual charge before it releases.

Although certain specific embodiments of the invention have been shown in the drawings and described in the foregoing specification it will be understood that the invention is not limited to such specific embodiment but is capable of modification and rearrangement without departing from the spirit of the invention and within the scope of the appended claims.

, What is claimed is: I

1. Ina telegraph system, an outlying station having a line, a central ofiice station having a terminating circuit for said line including separate repeating relays for directly repeating signals from said line and into said line, means controlled by one of said relays for disqualifying the other of said relays from repeating signals without estopping signal responsive operation of said other relay, and means controlled by the lastmentioned means for requalifying said other relay to repeat signals.

2. In a telegraph system, an outlying station having a line, a central ofiice station havin a terminating circuit for said line including separate repeating relays for repeating signals from said line and into said line, means controlled by one of said relays for disconnecting from one of the contacts of the other relay the normally connected source of signaling potential and for substituting on said contact the same potential and polarity as that appearing on the other contact of said other relay whereby to disqualify said other relay from repeating signals, and means controlled by the last-mentioned means for reconnecting the connected to said spacing contact and for substituting on said spacing contact the same potential and polarity as that appearing on the marking contact of said other repeating relay, and a polar relay connected between said spacing con tact and said substituted potential and operable upon closure of a circuit through the armature and spacing contact of said other repeating relay to current of the proper polarity for reconnecting to said spacing contact said normally connected source of signaling current.

4. In a telegraph system, an outlying station having a line, a central ofiicestation having'a terminating circuit for said line including separate repeating relays for repeating signals from said line and into said line, a relay operable by one of said repeating relays upon operatin to the spacing condition for disconnecting from the spacing contact of the other repeating relay a normally connected source of signaling current and for substituting on said spacing contact a connection of the same polarity and potential as that appearing on the marking contact of said other repeating relay, a biased polar relay having its operating winding, armature and the contact normally engaged by said armature serially connected between said pacing contact and said substituted connection, said polar relay being operable upon closure of a circuit through the armature and spacing contact of said other repeating relay to current of the proper polarity for reconnecting to said spacing contact said normally connected source of signaling current through said operating winding, armature and the other contact of said polar relay.

5. In a telegraph system, an outlying station having a line, a central office station having a terminating circuit for said line including separate repeating relays for repeating signals from said line and into said line, a relay operable by one of said repeating relays upon operation thereof to spacing condition for disconnecting ,from the spacing contact of the other repeating relay a source of signaling current normally connected thereto and for substituting a connection of the same potential and polarity as that appearing on' the marking contact ofsaid other repeating relay, a polar relay having its operating winding, armature and the contact normally enother repeating relay to current of the proper maintaining the operatin winding of said polar gaged by said armature connected in series between said spacing contact and said substituted connection and operable upon closure of a circuit through the armature and spacing contact of said polarity for reconnecting said normally connected source of signaling current to the spacing contact of said other repeating relay, and means including said source of signaling current for able by one of said repeating relays upon operation thereof to spacing condition for disconnecting from the'spacing contact of the other repeating relay a source of signaling current normally connected thereto and for substituting a connection of the same potential and polarity as that appearing on the marking contact of said other relay, a polar relay having its operating winding,

armature and the contact normally engaged by said armature connected in series between said spacing contact and said substituted connection, and operable upon closure of a circuit through the armature and spacing contact of said other repeating relay to current of the proper polarity for reconnecting said normally connected source of signaling current to the spacing contact of said other repeating relay, and means controlled by said control relay upon operation thereof for increasing the sen itivity of said polar relay.

7. In a telegraph system, an outlying station having a line; a central oifice station having a termination circuit for said line, a repeating relay for sending signals into said line, a repeating relay for receiving signals from said line, said receiving relay being operable to marking condition over a-circuit including a source of signaling current at said outlying station and the armature disconnecting from the spacing contact of said" sending relay a source. of signalin potentialnor mally connected to said contact andfor placing said spacing contact at the samelpotential'as the marking contact of said sending relay whereby to preclude saidreoperation of said spacingirelay' to marking.

8. In a telegraph system, anoutlying station. having a line, a central officestation having aterminating circuit for said line including separate repeatingrelays'for repeatingsignals from' said line and into said line, means controlledbyone of said relays for disqualifying th other of said relays from repeating, signals; means 3 controlled by said disqualifying means for requalify ing said other relay to repeat signalsand means" also controlled'by said disqualifying'means for delaying the'operation of said requalifying'means.

9. In a telegraph system; an outlying station having a line, a central oiiicestation' having a termination circuit forsaid line, a repeating relay for sending signals into' saidlline, a repeating relay for receiving signalsfro-m said line, a

control relay operable by said receiving relay upon operation thereof to'spacing for disconnecting fromthe spacingcontact of said sending relay a sourceof signaling potential normally connected-thereto and for substituting'on-said contact a connection of the same potential and polarity as that appearing on' the marking contact oi saidsending relay, a polar relay for'reconnecting to saidspacingcontact said normally connected source of signaling current, and a slow-release relay releasable by said control re"- lay for introducing the energizing circuit ofxsaid" polar relay between said substituted connection and said spacing contactwhereby to' render said polar relay operable upon" closure of a' circuit through the. armature and spacingv contact of said sending relay to currentof the proper polarity.

10. In a telegraph system, an outlying station having a line and a source of signaling current, means for connecting said source to said line, a central office station having! a terminating circuit for said line including a repeating relay for receiving. signals. from said line and a repeating relay for sending signals tosaid'line, said' receiving'relay being susceptible of operation. to spacing conditionupon disablement of saidconnectin-g means, a source of signaling current normally connected to the spacing contact of said.

sendingrelay, a supervisory relay in said terminating circuit energizable by signaling-current.

flowing in said line and susceptible of energization from said'sour'ce of signaling current'connected to said spacing contact upon operation of said sending relay to spacing condition with said. connecting means disabled, means controlled by controlled by said supervisory relay upon release.

thereof for displaying an indication of disable ment of said connectingimeans.

tta ns-- 1-1. lnatelegra'phsystem; a plurality'ofoufl lying stations each havingaline'; means Ior-con i said lines" including arepeatingrelay tbrreceiir ing signals from its'associated'line forretrans mission to anotherstation and a repeating" relay for sending toits associatedline signals'received' from another station, means at said-central'oificm station for interconnecting any two'ofsald'te'r' mination circuits to establish'a communication path between the outlying stations servedbysaid termination circuit, means operable" by each of" saidreceiving relays upon: operation thereofto" spacing'for disconnectingfrom the'spacing contact of the sending'relay a source of'si'gnaling" current'normally connected thereto and 'forsub stituting on said' spacing contact a connection having thesame potential and polarity asthat appearing on' the markingcontactof saidsending' relay, and means operable upon signal responsive operation of said sending, relay to' spacing condition: only while said outlying station connecting means remains operated for'reconnecting' said normally connected source ofsignalingcurrent to saidspacing contact;

12. In" a telegraph system; a" line, a; repeaterassociated with said line having" a relay' for" repeating'signals from said'line andi'a' relayfor' repeating signals into said line, me'ans controlled" by'one of said'relays fordisconn'ectingfrom' one of the contacts of the other relay'the normally connected source of' signaling potential and for substituting on said contact the same potential and polarity, as that appearingron'the other contact of said other relay whereby 'to disqualify said other relay, from repeating signals, and means controlled by the' last-mentioned means for reconnecting the normal'source of signaling, potential to the first-mentioned contact of'said other relay whereby to requalify saidother relay to repeat signals.

13; Irratelegraph system, two line sections; a repeater interconnecting saidiline sections having. relays for repeating signalsfrom each line section into the other line section, means controlled. by one of said relays for disconnecting from oneof the contacts of the other relay, the normally connected. source of.' signaling 2 potentialand for substituting on-saidcontactthe same potential and polarity. as. that. appearing on the other contact oflsaidiother. relay whereby to disqualify said other relay. from repeating. signals,v

and. means. controlled. by. the. last-.mentioned meansfor reconnecting. the normal source.of.'sig nalingpotential to the first-mentioned contact of said other relay whereby tmrequalify said. other. relay: to repeat signals.

14..In .a telegraph= system two line. sections. a repeater. interconnecting said line sectionsliavinga relay associated-with each.line section for directly repeati'ngsignalsinto the. other line sec.- tion, means. controlledby. one. of said! relays for depriving the other relay of". signal' repeating. capability. Without estopping signal. responsive operation of. said other relay; and" means controlled by'said last-mentioned means for 'restor ing signal. repeating capability to said" other re= Jay.

RICHARD B'f HEARN.- 

